In the field of audio circuits used in consumer goods such as radios, TV sets, voice recorders and the like, there has been an increasing demand for reducing costs by minimizing the silicon area requirement and number of discrete external components. At the same time, there has also been a concern about providing a sufficient amount of power, even at low supply voltages, as in the case of battery powered portable devices. Consequently, efforts have been directed towards simplification of audio amplifying stages for minimizing the occupied silicon area. In addition, efforts have been directed toward achieving satisfactory stability characteristics without requiring the use of external components, such as for example, RC compensation networks and the use of an output stage having the lowest voltage drop possible to maximize the power that may be delivered.
FIG. 1 shows a typical power amplifier used in audio devices. The output stage is formed with low voltage drop NPN transistors. Although the output stage is characterized by a relatively low voltage drop, the presence of two feedback loops, whose gain is difficult to control because they are tied to the maximum output current that may be delivered, requires the use of an external RC network to ensure adequate stability characteristics. Another integrated power amplifier that is widely used as an alternative to the structure of FIG. 1 is shown in FIG. 2. This amplifier is referred to as the OTA stage or amplifier, where OTA is an acronym for operational transconductance amplifier.
The amplifier in FIG. 2 is different from the amplifier of FIG. 1 because it uses a complementary pair of output transistors, Q1 and Q2. Another aspect which differentiates these two amplifiers is the way in which the quiescent bias current is fixed, i.e., the amplifier is idle. In this case, the biasing current is based upon the mirroring ratios in which the reference bias current I.sub.ref of the input differential stage is multiplied. The maximum current that may be delivered to the external load is therefore limited to the value given by the following expression: EQU I.sub.Max .about..beta..sub.1,2 I.sub.Ref.
There exists a need for an integrated amplifier to provide a high maximum output current with a minimum voltage drop through the output stage of the amplifier. This is in addition to satisfying simplicity requirements such as limiting the silicon area need for integration and having sufficient intrinsic stability characteristics to eliminate the need for using compensating external networks.